1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display, and more particularly to a method for crystallizing an amorphous silicon film in order to form a single crystalline silicon film having high uniformity and a superior device function.
2. Description of the Prior Art
Generally, a TFT (thin film transistor), which is a switching device used for a liquid crystal display or an organic light emitting display made from an organic electroluminescent material, is the most important component for achieving superior performance of flat panel displays.
Herein, mobility or leakage current of the TFT, which is a basis for judging performance of the TFT, may seriously relate to a state or a structure of a silicon film used as a material for an active layer providing a passage for an electric charge carrier. In a case of liquid crystal displays available from markets, most of active layers of the TFT are made from an a-Si (amorphous silicon) film.
However, an a-Si TFT employing an a-Si film as an active layer has low mobility of about 0.5 cm2/Vs, so problems may occur if all switching devices provided in the liquid crystal display are made from the a-Si TFT. That is, a driving device for a peripheral circuit of the liquid crystal display must be operated with a high speed. However, the a-Si TFT cannot satisfy a driving speed required by a peripheral circuit driving device, which means that the a-Si TFT is not adaptable for fabricating the peripheral circuit driving device.
Accordingly, when manufacturing the liquid crystal display, peripheral circuit driving parts, such as a driving circuit, various controllers, and a digital-analog converter, are integrated on a single crystalline silicon film in the form of devices in order to respond to a high driving speed required for driving the liquid crystal display. Also, the a-Si TFT has a switching function and a low leakage current characteristic, which are essentially required for ensuring superior image quality, so that the a-Si TFT is used as a pixel switching device.
Meanwhile, a poly-Si (poly silicon) TFT including a poly-Si film as an active layer has high mobility in a range of few tens cm2/Vs to few hundreds cm2/Vs, so the poly-Si TFT may be operated with a high driving speed corresponding to the peripheral circuit driving device. For this reason, if the poly-Si film is formed on a glass substrate, the peripheral circuit driving parts and the pixel switching device can be easily formed. Accordingly, an additional module forming process for forming peripheral circuits is not necessary. In addition, peripheral circuit driving parts may be formed while forming a pixel region, so a manufacturing cost for the peripheral circuit driving parts can be reduced.
Also, since the poly-Si TFT has high mobility, the poly-Si TFT can be manufactured with a compact size smaller than a size of the a-Si TFT. In addition, the driving device for the peripheral circuit and the switching device for the pixel region can be simultaneously formed through an integration process so that a micro design rule may be easily achieved. Therefore, the poly-Si TFT is very advantageous for achieving high resolution, which cannot be achieved by using the a-Si TFT LCD.
In addition, since the poly-Si TFT has a high current characteristic, the poly-Si TFT is adaptable for a driving device of an organic light emitting display, which is a next generation flat panel display. Recently, a study for manufacturing the TFT by forming the poly-Si film on the glass substrate and a study for applying the poly-Si TFT to the organic light emitting display have been actively researched.
For example, after depositing the a-Si film on the glass substrate, a predetermined heat treatment process is carried out with respect to the a-Si film such that the a-Si film is crystallized, thereby forming the poly-Si film on the glass substrate. In this case, the glass substrate may be deformed at a high temperature above 600° C., so a yield rate and reliability of articles are decreased.
Therefore, an ELA (excimer laser annealing) process capable of crystallizing only the a-Si film without causing thermal damage to the glass substrate is used as a crystallizing method for the a-Si film.
According to the ELA process, the a-Si film is melted through a laser irradiation process, and then, the melted a-Si film is crystallized while being solidified. While the a-Si film is being crystallized, grains are grown from crystal nuclei, which are randomly created while the a-Si film is being melted and solidified. At this time, the grains may be formed with various sizes in a range of few tens nm to few μm according to a laser irradiation condition.
FIG. 1a shows plan and sectional views for explaining a conventional ELA process, and FIG. 1b is a photographic view showing a microstructure of poly-Si achieved through the ELA process.
According to the ELA process, as shown in FIG. 1a, an excimer laser 13a irradiates onto an entire surface of an a-Si film 11 deposited on a glass substrate 10 with a thickness of about 100 to 2,000 Å according to a predetermined scan manner. At this time, a predetermined region of the a-Si film 11 receiving the excimer laser 13 is crystallized, so a poly-Si film 12 is formed. A buffer layer 14 is interposed between the glass substrate 10 and the a-Si film 11 in order to prevent a device characteristic from being lowered due to impurities derived from the glass substrate 10 while the excimer laser 13 is being irradiated onto the a-Si film 11.
The a-Si film 11, onto which the excimer laser 13 is irradiated, is partially melted or completely melted according to energy values of the excimer laser 13. A microstructure of the poly-Si film 12 achieved by crystallizing the a-Si film 11 is shown in FIG. 1b. 
Herein, a size of a grain existing in the poly-Si film 12 becomes enlarged as energy of the excimer laser increases if the a-Si film 11 has not been completely melted. Also, after the size of the grain in the poly-Si film 11 has become a maximum size, the size of the grain existing in the poly-Si film 12 becomes reduced if the a-Si film 11 is completely melted. Even though it is necessary for achieving the superior device characteristic to reduce crystalline faults, such as a grain boundary, if the size of the grain becomes enlarged, distribution uniformity for the grains may be deteriorated, so uniformity of the device characteristic is also deteriorated, thereby decreasing a yield rate and reliability of articles.
Accordingly, even though the superior device characteristic is an important factor for allowing the poly-Si TFTs to be manufactured in mass-production, a uniformity degree of the device characteristic must be ensured together with the superior device characteristic. Accordingly, when the poly-Si film crystallized through the ELA process is applied to articles, the poly-Si film having the grain of a limited size must be used for ensuring uniformity of the device characteristic.
However, in this case, the poly-Si TFT has inferior mobility due to the limited size of the grain. In addition, integration for peripheral circuits may be limited, so it is difficult to realize an SOG (silicon on glass) structure through the ELA process.
In addition, the ELA process has a limitation in view of the device characteristic, and has following limitations with regard to the process.
According to the ELA process, laser irradiation energy is unevenly formed in each shot. In addition, since about 20 to 30 μm of a profile section causing laser energy to decrease may exist around a laser beam having a size of about 300 to 400 μm, an overlapped ratio per one shot is maintained above 90 percent in order to ensure superior uniformity. For this reason, the laser beam repeatedly irradiates onto the same region by at least 10 times in order to completely crystallize the region, so the ELA process has a disadvantage in view of process efficiency and manufacturing costs.
Meanwhile, U.S. Pat. Nos. 6,322,625 and 6,368,945 (issued to James Im et., al) disclose a crystallizing method capable of ensuring a grain having a large size, high productivity and high uniformity. As shown in FIG. 2a, according to a method called an “SLS (Sequential Lateral Solidification)”, a predetermined mask 25 is aligned between a laser beam 23 and an a-Si film 21 so as to convert the laser beam into a required shape, thereby crystallizing the a-Si film 21. In a practical process, a predetermined optical system 25a is aligned between the mask 25 and the a-Si film 21, so that the laser beam 23 passing through the mask 25 is irradiated onto the a-Si film 21 while a size of the laser 23 is being reduced with a predetermined scale. In FIG. 2a, reference number 20 represents a glass substrate, and reference number 24 represents a buffer layer, respectively.
A two-shot SLS process is provided as a solution for realizing an SLS concept. FIG. 2b is a plan view showing a mask employed for the two-shot SLS process. As shown in FIG. 2b, the mask 25 for the two-shot SLS process includes slit arrays of two rows, which are aligned offset from each other.
According to the two-shot SLS process, a laser beam is primarily irradiated through a slit pattern 26, which is an open region formed in the mask 25 shown in FIG. 2b. In this case, as shown in FIG. 2c, a slit pattern region including a plurality of slits and formed in an a-Si film corresponding to the slit pattern 26 is crystallized. At this time, a crystallization process is carried out over the whole area of the slit pattern region by grains grown from edge sections of each slit of the slit pattern region to the center portion of each slit. Also, when grains grown from both edge sections of the slit make contact with each other at the center portion of the slit, the crystallization process has been finished. As a result, protrusions 210 are created at a center portion of a crystallization region due to collision between grains.
Meanwhile, such a phenomenon may be realized by allowing the slit to have a width of about 3 to 5 μm in such a manner that the grains grown from the edge sections of the slit can make contact with each other at the center portion of the slit, before spontaneous nucleation is generated at the center portion of the slit caused by solidification of silicon, which has been melted by the laser irradiation process. If the width of the slit is too wide, poly-Si is created around the center portion of the slit due to nucleation before the grains grown from one edge of each slit make contact with grains grown from the other edge of each slit at the center portion of each slit. Accordingly, the crystallization characteristic of the a-Si film is deteriorated, so the device characteristic and the degree of uniformity are lowered.
Hereinafter, a crystallization status of the a-Si film after the primarily laser irradiation has been carried out will be described. As is understood from a figure provided in a lower part of FIG. 2c, a lateral growth length Lg of the grain is a half of a width W of each slit provided in the slit pattern region, and crystallized regions are created. At this time, the protrusions 210 are constantly aligned with a predetermined interval Sp1. Each crystallized region includes a micro grain region 212 formed at a start point of the crystallization and grains 220 having long lengths extending to the protrusions 210. That is, after the primary laser irradiation has been carried out, a slit array pattern corresponding to a slit array formed in the mask 25 is formed in the crystallized region. Also, an interval between crystallized regions in the form of the slit array pattern is substantially identical to an interval d1 formed between slits of the slit pattern region formed in the a-Si film 21, which are partially crystallized.
Since a non-crystallized a-Si region may exist after the primary laser irradiation process has been carried out, a secondary laser irradiation process is carried out through the slit pattern 26 formed in the mask 25 shown in FIG. 2b in order to entirely crystallize the a-Si film. At this time, the secondary laser irradiation process is carried out by irradiating the laser beam onto the a-Si film after moving the substrate by a predetermined distance from the crystallized region formed through the primarily laser irradiation process. The moving distance for the substrate is identical to or less than a slit length L. Referring to FIG. 2b, the mask 25 has slit arrays of two rows, which are aligned offset from each other, so a secondary laser irradiation region may include an overlapped part of the non-crystallized region and the crystallized region formed through the primary laser irradiation process.
FIG. 2d is a view for explaining a crystallization status of the a-Si film after the secondary laser irradiation process has been carried out. A region melted during the secondary laser irradiation process, that is, the region formed between reference number 216a and reference number 216b, includes the non-crystallized region and a part of the crystallized region formed through the primary laser irradiation process. Reference numbers 216, 216a, and 216b represent a liquid-phase Si region melted through the secondary laser irradiation process and the remaining poly-Si region, that is, the region formed between reference number 216 and reference number 216b, which is crystallized through the primary laser irradiation process and still exists without being melted through the secondary laser irradiation process. Accordingly, after the secondary laser irradiation process has been carried out, grains may grow from a boundary formed between the liquid-phase Si region and the remaining poly-Si region into the melted region by utilizing seed grains, which have been formed when the primary laser irradiation process is carried out. If seed grains formed at both edge sections of the slit grow and make contact with each other at the center portion of the slit, the crystallization process has been finished. At this time, protrusions 217 and 217a are newly created in the same manner as the primary laser irradiation process.
FIG. 2e is a photographic view showing a microstructure in a poly-Si film achieved through the crystallization process after the two-shot SLS process has been carried out.
Such a two-shot SLS process may crystallize a predetermined region by irradiating the laser beam by two times. In addition, such a process can be repeatedly performed while moving a stage of SLS equipment, on which a substrate is placed, thereby crystallizing the a-Si film formed on the entire surface of the substrate.
The two-shot SLS process is able to rapidly crystallize a broad region regardless of a size of the substrate, and has a wide process window. In addition, the two-shot SLS process can fabricate grains having a size larger than a size of grains fabricated through the above-mentioned ELA process and can control the microstructure of the crystallized poly-Si film.
However, since the poly-Si film achieved through the two-shot SLS process is not a single crystalline Si film, it has a limitation for improving the characteristics of a TFT. In addition, an uniform alignment of the poly-Si grains aligned in a row may cause a disadvantage if the alignment of the poly-Si film and the single crystalline Si film does not match with a pixel alignment and a peripheral circuit alignment formed on a glass substrate.
In addition, the two-shot SLS process may cause a minute difference between crystallized parts depending on energy fluctuation per a laser shot. Thus, the microstructures between protrusions are slightly different from each other, resulting inferior uniformity.
As a result, the poly-Si film achieved through the two-shot SLS process has limitations for achieving a superior device characteristic adaptable for an SOG structure. In addition, since the TFTs for a crystallized region, a pixel section and a peripheral circuit must be aligned in match with each other, a design scheme for the poly-Si film may be limited.